NEW YORK — (CNN) Donald Trump must rule out a third party bid before October if he wants to compete in South Carolina’s Republican primary, a crucial test in the nominating contest.

Trump has repeatedly refused to rule out a third party candidacy, noting that he could use the threat of an independent bid as leverage, but he cannot appear on the South Carolina primary ballot unless he pledges to support the GOP nominee in the general election.

Candidates must file their candidacy with the South Carolina GOP by Sept. 30, submitting a form that includes a signed pledge stating to “hereby affirm that I generally believe in and intend to support the nominees and platform of the Republican Party in the November 8, 2016 general election.”

Trump, who is campaigning in South Carolina on Thursday and snagged 30% of support in a poll released Tuesday, has yet to sign the pledge.

Four candidates — Sen. Marco Rubio of Florida, former Florida Gov. Jeb Bush, Ohio Gov. John Kasich and retired neurosurgeon Ben Carson — have already filed the form and Wisconsin Gov. Scott Walker, Sen. Ted Cruz of Texas and South Carolina Sen. Lindsey Graham are filing this week, South Carolina GOP Chairman Matt Moore said in a statement Tuesday.

Trump is the only Republican presidential candidate to refuse to pledge not to launch an independent campaign if he cannot secure the GOP’s presidential nomination — a fact that was put into relief during the first GOP primary debate earlier this month when Trump was the only candidate to raise his hand when the moderators raised the question at the top of the debate.

CNN has reached out to the Trump campaign for comment.

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